Mustang Diff acting up AGAIN

[Barkleymut]

My 07 Mustang GT had a really bad howling/whine a coupke of years ago. Went thru several NAPA reman rear ends before finally pulling a full rear end off a junkyard unit. That was a year ago and all has been good. This week I starting hearing a very slight howl again when I let off the gas at highway speed. Before I took this unit (which i think had around 120k miles on it) i pulled the cover and the fluid that came out was definately not original. I put in Valvoline 75w140.
THat was maybe 7 or 8k miles ago.
Planning on pulling the cover again and replacing this oil. What is the best oil for this application? I do not drive this car hard. Would something like Amsoil Severe gear 75-110 or is there an HPL Gear oil that would give me the best chance at keeping this rear end from getting worse?

 

[97prizm]

Does it have a limited slip in it?

 

[Barkleymut]

Yes

 

[97prizm]

I've always used Royal Purple in my Mazda rx-7 racecar as it has friction modifiers for lsd already in it.

 

[supton]

What are the odds of two diffs going bad? Are they known to be weak, or was this just unfortunate coincidence?

Is there a chance that this coasting noise is something else?

 

[97prizm]

"If the cause of the noise is inside the differential, this can be due to either tight-meshing pinion gears, or too much backlash on these gears. Another cause could be worn out differential-case bearings, and these can also cause noise when turning".

 

[97prizm]

I'd check a mustang forum. I know Ford cheaped out on the previous GT mustang when they used Chinese manufactured transmissions. The shift forks failed repeatedly. The GT350 and GT350R had USA made transmissions that had no issues.

 

[skyactiv]

Do you have the hearing range of a dog? Some people hear things most of the population can't and it could be a non issue.

 

[Barkleymut]

Ha, my wife would argue that I am legally deaf. The whine is very slight, but I swear it was not there 2 weeks ago. As for the backlash and bushings, I dont understand enough about how differentials work to even begin to check all that stuff. Really do not want to try and find anyone in this mechanically inept town to "fix" the problem. I have worn out the internet searching for this problem and it doesnt seem to be too common. Car only has 114k miles and runs perfect now.

 

[t1snwrbrdr12]

8.8 is a good rear, I ran mine to over 550hp at the wheels with no problem. It was a 31 spline truck diff with moser axles, but still a stock diff.

There’s a chance a u joint is going out, that can be very similar to a gear noise when you first catch it…but sounds like you’re just having no luck.

 

[Dave Hess]

Did you add the small bottle of Ford friction modifier to the gear oil ?

 

[Chris142]

Napa sells remanned rear ends?

 

[meep]

my 2018 f150 started differential whine around 40k miles. I knew it was too faint for the dealer to lift a finger. I didn’t put many miles on it for a while and it’s up to 60k. It whines every morning when the temps are below 50. It doesn’t seem to be getting worse.

I had a Chevy with real rear end growl after it was rear ended. We drove it that way for years before getting it fixed. They had no trouble setting it back up right. Diffs are tough.

im of the opinion ford just has sloppy QC on their diffs, but not necessarily any worse than Jeep, from personal experience.

 

[The_Eric]

I sort of remember your problems when you posted about this last - I seem to remember that you ended up going with a smaller rear axle? Like the 7.5 from a non GT?

 

[Barkleymut]

Did you add the small bottle of Ford friction modifier to the gear oil ?

Yes...even though the valvoline label made it seem like it was not needed

 

[Barkleymut]

I sort of remember your problems when you posted about this last - I seem to remember that you ended up going with a smaller rear axle? Like the 7.5 from a non GT?

Looked at a 7.5 but ened up with an 8.8

 

[Barkleymut]

Napa sells remanned rear ends?

The "napa authorized service center" aka transmission shop sold and installed 5...yes that is correct, 5 rear ends. They finally gave up and asked what I wanted to do. Finally found a junkyard unit and crossed my fingers. Either way I am not doing anything to this unit except play with different gear oils to figure out which one makes the sound go away.

 

[Jason2007]

Could also be the bearing in the two piece driveshaft.

 

[2strokeNorthstar]

Why would you not just have it rebuilt from a good driveline shop in the first place? Probably $1000-$1500 and better than any reman

 

[DuckRyder]

To be frank I do not think gear oil is going to fix a noise but i guess its a low cost hail Mary.

No way id have swapped an original rear end with a napa rebuild but i suppose that ship sailed.

my bet is you're gonna have to get a reputable person to build it, or learn to do it yourself.